GSM Radio Modem Design: From Speech to Radio Wave
Earlier in Chapter 5, we discussed about the various medium access techniques. In a GSM system, a combination of time and frequency division multiple access (TDMA and FDMA) techniques are used to multiplex the air medium among the users. For uplink and downlink separation, GSM uses a frequency division duplex (FDD) technique. So the uplink and downlink frequencies are different. As mentioned earlier, GSM 900 was the first GSM system to be deployed, and uses a 25 MHz frequency band (935–960 MHz) in the downlink and another 25 MHz frequency band (890–915 MHz) in the uplink direction (Figure 7.1). The uplink and downlink bands are separated by 45 MHz. Again, FDMA divides the frequency bandwidth of the 25 MHz (maximum) in each direction (uplink and downlink) into 125 carrier frequencies, where each carrier frequency has a 200 kHz bandwidth. Although GSM 900 is most popular, currently there are several other types of networks available in the world using a GSM standard at different frequency bands, and these are given in Table 7.1.
As mentioned earlier, in the GSM 900 system, the total available number of uplink and downlink carriers = 25 MHz/200 kHz = 125. However, the last one cannot be used, as it is kept as a guard band with other adjacent wireless systems. So the total available carriers in each direction = 124, and of these again the top and button ones are used for additional guard band purposes with other wireless ...